High-quality adhesives are increasingly used in the manufacture of vehicles and attachments or also of machines and devices instead of or in combination with conventional joining methods such as screws and bolts, rivets, punching, or welding. When adhesively bonding structural parts, high strength and impact strength of the adhesive are of prime importance.
In vehicle manufacture and in the field of vehicle repair, thermosetting adhesives, which are usually cured at temperatures above 60° C. and sometimes up to 170° C., have established themselves. However, their use is often disadvantageous, for example in the repair of vehicles in repair shops where the equipment required for heat curing is not available. Moreover, the repair of vehicles—contrary to the manufacture thereof—is carried out on fully equipped vehicles so that heat-sensitive materials could be affected.
Consequently, adhesives which can already be cured at room temperature, in particular two-component epoxy resin adhesives, gain in importance. However, these often do not have the same mechanical properties as the thermosetting adhesives, which offers potential for further improvements.
In general, epoxy adhesives are characterized by high mechanical strength, in particular high tensile strength. However, conventional epoxy adhesives are in most cases too brittle when subjecting the adhesive joint formed therefrom to an impact loading and are thus far from meeting the requirements in particular of the automotive industry under crash conditions involving both high tensile and peel stresses. In this regard, the strengths at high, but in particular also at low temperatures below −10° C. are insufficient.
By using various methods, it has been tried to improve the impact strength of epoxy adhesives.
According to the literature, essentially two methods for reducing the brittleness of epoxy adhesives and thus improving the impact strength have been proposed: on the one hand, the objective can be achieved by additions of at least partially cross-linked high-molecular compounds such as latices of core/shell polymers or other flexibilizing polymers and copolymers. On the other hand, a certain increase in toughness can be achieved by introducing soft segments, for example, by modifying the epoxy components as required.
WO 2009/025991 A1 describes epoxy resin adhesives for use in vehicle manufacture that cure at relatively low temperatures. The structural adhesives disclosed there have three main components, namely from 15 to 50% by weight of a primary or secondary amino group terminated polyether, from 4 to 40% by weight of a primary or secondary amino group terminated rubber, and from 10 to 30% by weight of a primary or secondary amino group terminated polyamide having a melting temperature not exceeding 50° C. Moreover, this composition additionally contains at least one impact strength modifier.
However, the described two-component epoxy resin compositions have room for improvement in terms of impact strength.